This invention relates in general to electrophotography and more specifically, to an electrophotographic imaging member and process for using the imaging member.
In the art of electrophotography an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated, duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the ground plane of many modern photoconductive imaging members must be highly flexible, adhere well to flexible supporting substrates, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles.
One type of ground plane which is gaining increasing popularity for belt type photoreceptors in vacuum deposited aluminum coated with two electrically operative layers, including a charge generating layer and a charge transport layer. However, aluminum films are relatively soft and exhibit poor scratch resistance during photoreceptor fabrication processing. In addition, vacuum deposited aluminum exhibits poor optical transmission stability after extended cycling in xerographic imaging systems. This poor optical transmission stability is the result of oxidation of the aluminum ground plane as electric current is passed across the junction between the metal and photoreceptor. The optical transmission degradation is continuous and, for systems utilizing erase lamps on the nonimaging side of the photoconductive web, has necessitated erase intensity adjustment every 20,000 copies over the life of the photoreceptor.
Further, the electrical cyclic stability of an aluminum ground plane in multilayer structured photoreceptors has been found to be unstable when cycled thousands of times. The oxidies of aluminum which naturally form on the aluminum metal employed as an electrical blocking layer prevent charge injection during charging of the photoconductive device. If the resistivity of this blocking layer becomes too great, a residual potential will build across the layer as the device is cycled. Since the thickness of the oxide layer on an aluminum ground plane is not stable, the electrical performance characteristics of a composite photoreceptor undergoes changes during electrophotographic cycling. Also, the storage life of many composite photoreceptors utilizing an aluminum ground plane can be as brief as one day at high temperatures and humidity due to accelerated oxidation of the metal. The accelerated oxidation of the metal ground plane increases optical transmission, causes copy quality nonuniformity and can ultimately result in loss of electrical grounding capability.
After long-term use in an electrophotographic copying machine, multilayered photoreceptors utilizing the aluminum ground plane have been observed to exhibit a dramatic dark development potential change between the first cycle and second cycle of the machine due to cyclic instability, referred to as "cycle 1 to 2 dark development potential variation". The magnitude of this effects is dependent upon cyclic age and relatively humidity but may be as large as 350 volts after 50,000 electrical cycles. This effect is related to interaction of the ground plane and photoconductive materials. Another serious effect of the aluminum ground plane is the loss of image potential with cycling at low relative humidity. This cycle down voltage is most severe at relative humidities below about 10 percent. With continued cycling, the image potential decreases to a degree where the photoreceptor cannot provide a satisfactory image in the low humidity atmosphere.
In Japanese Patent Publication No. J5 6024-356 to Fuji Photo Film KK, published Mar. 7, 1981, an electrophotographic photoreceptor is described comprising a conductive support, an inorganic amorphous silicon photosensitive layer which produces a charge carrier by photo-irradiation, and a charge blocking layer between the conductive support and the inorganic amorphous silicon photosensitive layer, the charge blocking layer forming a barrier against electric charge carriers. The charge blocking layer comprises an insulating or semiconductive material such as SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2, TiO.sub.2 or an organic polymer such as polycarbonate, polyvinylbutyral, etc. These charge blocking layer materials are intended to block electrons into the inorganic amorphous silicon photosensitive layer. Although not disclosed in this Japanese Patent Publication, it should be noted that charge blocking layer materials suitable for blocking electrons into an inorganic amorphous silicon photosensitive layer may not necessarily be suitable for blocking holes into an organic hole generator layer. To be operable, these blocking layers must not block holes from the positively charged inorganic amorphous silicon photosensitive layer to the conductive support. For example, an Al.sub.2 O.sub.3 film having a thickness of several hundred angstroms utilized as a blocking layer caused dark development potential cycle down, with accompanying dark decay, of a negatively charged multilayer structured photoreceptor comprising conductive ground plane, blocking layer, charge generating layer and a hole transport layer.
In some multilayered photoreceptors, the ground plane is titanium coated on a polyester film. The titanium coating is sputtered on the polyester film in a layer about 175 angstroms thick. The titanium layer acts as a conductive path for electrons during the exposure step in the photoconductive process and overcomes many of the problems presented by aluminum ground planes. Photoreceptors containing titanium ground planes are described, for example, in U.S. Pat. No. 4,588,667 to Jones et al. The entire disclosure of this patent is incorporated herein by reference. Although excellent toner images may be obtained with multilayered photoreceptors having a titanium ground plane, it has been found that charge deficient spots form in photoreceptors containing titanium ground planes, particularly under the high electrical fields employed in high speed electrophotographic copiers, duplicators and printers. Moreover, the growth rate in number and size of newly created charge deficient spots and growth rate in size of preexisting charge deficient spots for photoreceptors containing titanium ground planes are unpredictable from one batch to the next under what appear to be controlled, substantially identical fabrication conditions. Charge deficient spots are small unexposed areas on a photoreceptor that fail to retain an electrostatic charge. These charge deficient spots become visible to the naked eye after development with toner material. On copies prepared by depositing black toner material on white paper, the spots may be white or black depending upon whether a positive or reversal image development process is employed. In positive image development, charge deficient spots appear as white spots in the solid image areas of the final xerographic print. In other words, the image areas on the photoreceptor corresponding to the white spot fails to attract toner particles in positive right reading image development. In reversal image development, black spots appear in background areas of the final xerographic copy. Thus, for black spots to form, the charge deficient spots residing in background areas on the photoreceptor attract toner particles during reversal image development. The white spots and black spots always appear in the same location of the final electrophotographic copies during cycling of the photoreceptor. The white spots and black spots do not exhibit any single characteristic shape, are small in size, and are visible to the naked eye. Generally, these visible spots caused by charge deficient spots have an average size of less than about 200 micrometers. These spots grow in size and total number during xerographic cycling and become more objectionable with cycline. Thus, for example tiny spots that are barely visible to the naked eye can grow to a size of about 150 micrometers. Other spots may be as large as 150 micrometers with fresh photoreceptors. Visual examination of the areas on the surface of the photoreceptor which correspond to the location of white spots and black spots reveals no differences in appearance from other acceptable areas of the photoreceptor. There is no known test to detect a charge deficient spot other than by forming a toner image to detect the defect.